Three conjugated polymers comprised of 9,9-dioctylfluorene and 2,2‘-bipyridine, which are alternatively linked by the C−C single bond (P1), vinylene bond (P2), or ethynylene bond (P3), have been synthesized via the Suzuki reaction, the Wittig−Horner reaction, and the Heck reaction, respectively. The optical, electrochemical, and other physical properties of the polymers are dependent on the linkers. The polymer linked by the C−C single bond exhibits a much larger Stokes shift compared with the other two polymers, indicative of higher extended and rigid backbone conformations in the polymers linked by the vinylene and ethynylene bonds. All the three polymers are sensitive to the existence of a variety of transition metal ions due to the chelation between the 2,2‘-bipyridyl moieties and the metal ions. For the metal ions which have moderate and weak coordination ability with the 2,2‘-bipyridyl moieties, an obvious difference in response sensitivity is observed among the three polymers: P1 has the highest sensitivity, which is followed by P2, and P3 always exhibits the lowest sensitivity. The different sensing sensitivity is attributed to the different backbone rigidity of the three polymers, which is caused by the three different linkers. The results suggest the use of C−C single bond linker in the molecular design toward the 2,2‘-bipyridyl-based conjugated polymer chemosensors for achieving higher sensing sensitivity.
The fast-growing motion capturing/monitoring technique has raised a great demand for flexible strain sensors. For capturing complex motions (e.g., facial motion), both the strain amplitude and direction should be accurately detected. Although some reported sensors based on anisotropic conductive networks are proved to show accurate localization of strain directions, it is still a great challenge to achieve both high sensitivity and a high sensing range in these designs. Here, a self-assembled Ti 3 C 2 T x MXene film with parallel and periodic wrinkles is fabricated on a stretchable poly(dimethylsiloxane) substrate for constructing multidirectional strain sensors. During stretching, relative slip and crack will occur between the stacked MXene nanosheets, which contribute to high structural sensitivity in the MXene film. Meanwhile, the wrinkled structure contributes to high stretchability. As a result, the sensor based on the film with one-dimensional periodic wrinkles shows a large sensing range (>50%) and a gauge factor of 45. Furthermore, the sensor can accurately detect both the strain amplitude and direction by using the MXene film with two-dimensional wrinkles. It shows distinguishable electrical responses when detecting different-amplitude human/robot motions such as joint bending and walking. Additionally, the directions in complex human motions (e.g., facial motion) can also be well-tracked. This work provides an effective strategy to detect multi-directional motions.
A series of well-defined conjugated-liquid crystalline (LC) block copolymers containing oligofluorene and side-chain liquid crystalline polymers with cyanobiphenyl moieties were successfully synthesized by atom transfer radical polymerization. The block copolymers were prepared with number-averaged molecular weights (M n) ranging from 8000 to 16 000 and narrow molecular weight distribution less than 1.20. The chemical structures of these block copolymers were confirmed by 1H NMR, 13C NMR, and FTIR studies. All of the block copolymers exhibited the smectic mesophase as illustrated by differential scanning calorimetry, polarized optical microscopy, and wide-angle X-ray diffraction. A bilayer structure of mesogens was formed in the smectic layer of block copolymers with a thickness of 3.5 nm. The isotropization of the smectic phase increased with the molecular weight and leveled off at M n = 14 000. The optical properties of these block copolymers in solution and solid-films were investigated comparatively by UV spectroscopy, photoluminescence, and photoluminescent excitation characterization. The results suggest that energy transfer from the LC mesogens to the conjugated oligomer occurs both in dilute solution and in the solid state, which was more efficient in solid state due to higher local chromophore density.
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